1. Field of the Invention
The present invention relates to fluid control valves and more particularly to an improved seat or seal for such valves, particularly valves having a pivotable valve closure element such as a butterfly or disc valve.
2. Prior Art
Valves employing pivotable closure assemblies, such as butterfly valves, have found wide use in industry for fluid handling and control. Generally, such valves, particularly butterfly valves, have been used for controlling the flow of various fluids at ambient or moderate temperatures and pressures, since the materials commonly used in the seats of such valves tend to flow or extrude at elevated temperatures allowing the valve to leak. Additionally, many such materials, e.g. certain types of rubbers, are very susceptible to chemical attack by certain fluids and thus are unacceptable for use in valves used to handle such fluids.
There are various polymeric materials which are quite inert in the sense that they are resistant to chemical attack and degradation and, in addition, can withstand elevated temperatures. Notable among such polymers are the fluorocarbon resins or polymers such as, for example, polytetrafluoroethylene, e.g. Teflon. While the fluorocarbon polymeric materials possess excellent resistance to chemical attack and can withstand elevated temperatures, they possess little or no elasticity or resiliency and tend to "cold flow" once subjected to repetitive loading. Such polymers lack "memory" and thus when compressed, repeatedly or for extended periods, fail to return to their original configuration. Indeed, valve seats made of Teflon or like materials, after being stressed sufficiently and repeatedly, will become distorted due to cold flow to the point that they will not seal effectively and, therefore, allow the valve to leak.
It is known to provide rotary or pivotable valves having seats in which a thin coating of Teflon or similar material is bonded to a rubber backing to provide a valve in which the seat is resistant to chemical attack and, to a certain extent, can withstand elevated temperatures but nonetheless will not undergo cold flow due to the resiliency of the rubber backing. It is also known to provide rotary valve seats in which a Teflon seat is biased radially inwardly by means of a spring to retard cold flow.
In seat designs using springs or elastomers e.g. rubber, to provide memory to the Teflon seat, interference between the disc and the valve seat begins long before the disc is moved to the fully closed position and continues to increase as the disc moves to that position. This disc/seat interference produces accelerated wear, particularly in the area of the disc hubs, i.e. at the diametrically opposite points of the disc where it is rotatably journaled through the seat. In such prior art valves, seating torque and hence actuator sizing becomes a factor since significant torque is encountered well before the disc moves to the fully closed position. These problems may be partially alleviated through the use of double offset disc designs which may prevent interference between the disc and seat for up to 60.degree.-70.degree. of the disc travel from open to closed position. However, the interference which occurs during the last 20.degree.-30.degree. of travel still causes localized seat wear problems.